Documentation for JIRA 4.2. Documentation for other versions of JIRA is available too. 
In order to rename the "Priorities" text in the issue navigator, you may need to edit the following properties file in the language pack:
com/atlassian/jira/web/action/issue/IssueNavigator_en.properties
The following property value in the Issuenavigator_en.properties:
navigator.filter.matchingpriorities
The language pack is a JAR file located in <jira-install>/atlassian-jira/WEB-INF/lib. For more information about Customizing Text in JIRA, please refer to Customizing Text.
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